This invention relates to arc welding electrodes for producing weld metal having low amounts of hydrogen therein. More particularly, this invention relates to an improvement for so-called "low hydrogen" electrodes. In the improvement the flux coating contains a source of barium or cesium and the coating is bound together by a hydrolyzed organic silicate binder. The combination of the barium or cesium and the binder reduce the amount of hydrogen introduced into the weld metal. Additionally, the amount of oxygen found in weld metal of high strength steels is reduced.
In the art of welding, much prior effort has been expended in developing flux compositions of the type having predetermined flux components intended to perform in predetermined manners. A large number of compositions have been developed for use as fluxes in arc welding both for use generally as welding fluxes and for use as a coating on a metallic core. Examples of such flux compositions are set forth in detail in the patents to Lee, U.S. Pat. No. 2,432,773, and Mathias, U.S. Pat. No. 2,435,504.
Binders are used in such fluxes and in electrode coatings to hold the coating together and to maintain the desired shape of the electrode coating about the metallic core during normal handling. In general, the binders which are most predominantly used in the prior art are either sodium silicate or potassium silicate. Such binders have been particularly useful because they do not decompose under conditions of use and because they provide adequate strength characteristics in the quantity added to the flux composition for the high rate of extrusion used in the manufacture of such electrodes. In addition, the specific properties of either potassium silicate or sodium silicate makes each attractive for the manufacture of welding electrodes. For example, the drying characteristics are such that the liquid silicates used in welding electrodes as binders become hard films through the loss of water. By way of a specific example, one grade of sodium silicate with a ratio of SiO.sub.2 /Na.sub.2 O of 3.22 has a water content of 62.4 percent and a viscosity of 1/8 poises at 20.degree. C. The viscosity rises to 72 poises with a 6 percent weight loss by evaporation and to over 9000 poises with a 12 percent weight reduction of the original weight. When the evaporation totals about 14 percent of the original weight, the viscosity is about 40,000 poises and at this point the silicate, for practical purposes, has set. Further dehydration brings the silicate to a rigid condition.
The amount of moisture retained by the silicate film is governed primarily by the temperature to which it is subjected. It is known that room temperature air-drying of the silicate is not adequate for films or bonds that are to be used in welding. In some of the formulations known to the prior art for covered electrodes, materials are added which react with the silicate at high temperatures. For example, kaolin or other minerals which decompose by heat may restrict treatments to less than about 260.degree. C. (about 500.degree. F.).
The sodium and potassium silicates have been particularly useful because their properties provide characteristics which are essential in the manufacture of covered electrodes. In general, the practical approach to the use of silicate binders has been to determine the grade which is best suited for the manufacturing operation and to control the quality of the covered electrode by maintaining the properties of the binders. With the addition of liquid sodium silicate to a dry powder formulation, the resulting dough can be kneaded to a consistency that is appropriate for subsequent extrusion. The mass of kneaded dough is formed into "slugs" which facilitates handling during the time of storage and loading of presses for the extrusion operation. At present, a substantial portion of commercially produced coated electrodes for mild steel are produced by the extrusion process.
The plasticity of the coating batch is controlled somewhat by the silicate addition but may also be influenced by other plastic ingredients such as raw clay or bentonite which may be added or combined with non-plastic ingredients such as silica or calcined clay. As the electrodes are extruded, the electrode becomes reasonably solid and resists flattening as soon as the electrodes leave the die and fall on a conveyor belt. Drying is carried out at a low temperature beginning at about 100.degree.-150.degree. C. (about 200.degree.-300.degree. F.) with controlled humidity in order to obtain uniform drying of the coating without cracking. This drying step is followed by one or more higher temperature drying steps at a lower humidity depending upon the nature of the covering. The moisture content of the completed coating of the electrode will range from less than 0.1 percent in some low hydrogen electrode types to as high as 3 to 6 percent in a cellulose type of electrode. In electrodes of the high cellulose type (for example, of the class normally referred to in the art as E6010 and E6011) which produce ductile weld metal with a minimum of 60,000 pounds per square inch tensile strength, the use of water silicate binders can be particularly appropriate since the product may contain 3 to 4 percent moisture.
However, in higher strength, low hydrogen type electrodes these binders have not been satisfactory for at least several reasons. First, the drying of low hydrogen electrodes requires a high temperature treatment in order to drive off as much of the moisture as is necessary to meet the applicable specification. Second, the maintenance of this degree of dryness has been important in the welding of higher strength materials and such maintenance necessitates careful handling to avoid hygroscopic moisture pickup during the shop use of these electrodes. Although moisture pickup has not been particularly troublesome in coatings for lower strength weld metal, the hygroscopic characteristics of the present day low hydrogen coatings has made it mandatory to use heated ovens to maintain the dryness and restrict the pick up of moisture. In higher strengths, the hygroscopic nature of the coatings has been particularly damaging since, for example, in the EXXX18 type of electrodes, the moisture content must be kept at a level below 0.2 percent. Production facilities for producing such electrodes have the capability of reducing the moisture content to a level of less than 0.1 percent and in some cases to less than 0.05 percent.
For a low hydrogen flux composition having predetermined flux components (for example, those of the Lee and Mathias patents mentioned above) the sodium silicates and potassium silicates used as binders usually make up about 10 percent of the weight of the flux for any given electrode. When DC electrical power is used, the sodium silicate binder is used for a number of reasons. For example, the stability of the arc is not a significant problem with direct current. The sodium silicate is not quite as hygroscopic as potassium silicate and sodium silicate will dry to a lower moisture content than the potassium silicate. Thus, sodium silicates have been widely selected for low hydrogen electrodes. On the other hand, potassium silicate is normally selected when the electrode is used with an AC power source.
Because of the rigid moisture control required for low hydrogen electrodes, it has been a significant problem to many fabricators to prevent an excessive hygroscopic pickup of moisture on low hydrogen electrodes. By way of an example of the scope of the problem, it has been said that on a hot humid day, on the order of 90.degree. F. at 90 percent relative humidity in a shipyard, an exposed low hydrogen electrode will pick up enough moisture in about twenty minutes to exceed military specification standards. Thus, a close control must be maintained over the use and storage of low hydrogen electrodes.
A number of articles have been written and studies conducted on the storage and control of low hydrogen electrodes. At least one investigator has determined that the low hydrogen electrodes could not be successfully rebaked at low temperatures so that the consensus derived from the work of many investigators is that the most appropriate way to avoid hydrogen absorption by the weld metal is to keep the moisture content of the electrode coating to a minimum in the first instance. As a result, stringent controls have been placed on the moisture levels of the low hydrogen electrode. For example, the specified maximum moisture content of low hydrogen electrodes varies from a high of 0.6 percent H.sub.2 O for E60 or E7018 electrodes to a low of 0.1 percent H.sub.2 O for an E14018 type. These values are quite low when compared to other electrodes such as the EXX10 or the EXX11 series which may contain over 5 percent H.sub.2 O. For purposes of this disclosure, and in keeping with the art, "moisture level" or content is the percent, by weight, of H.sub.2 O in the flux coating.
It is thus clear that the lowering of the moisture level has created significant problems both for the manufacturer and the consumer in order to take extra precautions with the low hydrogen electrodes to insure that their moisture content does not rise above the required maximum levels.
Such precautions have included the steps of holding the electrodes at elevated temperatures to protect them from moisture absorption and rebaking the electrodes to recondition the electrode coating if the moisture content rises above the recommended level. Some electrode manufacturers even specify that the electrodes should be held in the oven at approximately 300.degree. F. until they are used and that if the electrodes are allowed to reach moisture levels which are too high, most manufacturers recommend that the electrodes be rebaked for one hour at about 700.degree. F.) to about 750.degree. F.
The redrying temperature depends on the composition and thickness of the covering. Coverings which contain organic material are usually redried at temperatures below the charring point (for example, about 250.degree. F.) whereas inorganic coverings, such as the low hydrogen types, are redried at temperatures up to 850.degree. F. It has been important in the art to follow precisely the drying procedures prescribed by the manufacturer for specific electrodes; otherwise, the electrodes may become unusable. For example, specified oven temperatures must be maintained after drying and electrodes should not be removed from holding ovens for more than several hours before being used; otherwise, redrying may be required.
The moisture content of low hydrogen coverings, for example, of the E7015, E7016, E7018 and E7028 electrodes should be kept below 0.4 percent for the former two and less than 0.6 percent moisture content for the latter. Preferably, the moisture content should be maintained below 0.3 percent. If the moisture content is significantly above these values, underbead cracking is likely and other undesirable effects may result. The criticality of monitoring and maintaining the moisture content of low hydrogen electrodes is underscored by the observation that a safe practice, followed by many fabricators, is to return all unused low hydrogen electrodes after either a two hour exposure or a working shift to a redrying oven maintained at 250.degree.-350.degree. F. for at least eight hours before reissuing them for use. Studies have shown that significant amounts of moisture are absorbed by the coverings on low hydrogen electrodes exposed for various periods of time.
While the undesired presence of hydrogen in weld metal for high strength steels has been of concern to the welding art, another source of concern is the undesired presence of oxygen. As pointed out by Heuschkel in The Welding Journal, 46 (2), pp. 74-s to 93-s (1967), the presence of oxygen of about 0.050% has an adverse effect on the impact properties of weld metal. Thus, it is desirable to reduce oxygen content to a value of less than about 0.030% in the weld metal of high strength steels.
Applicants acknowledge that ethyl silicate, barium and cesium are mentioned in the prior art of arc welding. See U.S. Pat. No. 2,052,400 to Moritz et al; U.S. Pat. No. 3,078,193 To C. E. Jackson; Pashchenko et al, Automatic Welding, pp. 62-62, April 1967; Hummitsch, Schweisstechnik, pp. 84-89, July 1948; C. E. Jackson, The Welding Journal, April-June 1960. However, none of this prior art teaches the improved combination of the present invention.
In view of the above background, it is an overall object of the present invention to provide an improvement in low hydrogen arc welding electrodes.
Another object of the present invention is to provide an improvement in such electrodes which are used for welding high strength steels.
Further, an object of the present invention is to minimize the amount of hydrogen introduced into weld metal from the flux covering and binder of an arc welding electrode.
Still further, an object of the present invention is to minimize the amount of oxygen introduced into weld metal for high strength steels.
These and other objects and aims of this invention will become apparent from a study of the written description of the invention.